Overhead vs Underground Water Tanks

Most Indian homes end up with both, doing different jobs. The underground sump is the bulk store: it fills from intermittent municipal supply whenever pressure is available, holds the larger volume (commonly 2–5× the overhead tank), keeps water cooler, and survives storms. Its costs are excavation, a pump, and harder leak detection.

The overhead tank is the pressure source: gravity feed means showers and taps work in a power cut, and a float valve plus pump automation keeps it topped from the sump. Its constraints are structural (water is 1 kg per litre — a 2000 L tank is two tonnes on the roof slab or staging), sun exposure (algae in translucent tanks; buy opaque, UV-stabilised), and visible plumbing. Sizing the slab or staging platform? A square footage calculator gives you the area and material quantities.

Sizing rule that works: overhead tank ≈ one day of household demand (persons × 135 L per IS 1172), sump ≈ two to three days. That way the pump runs once or twice a day, supply interruptions are bridged by the sump, and the roof load stays modest. Use the sizing calculator for your numbers.